Brake fluid is used to transmit the pressure exerted on a brake pedal, through hydraulic lines, to the cylinders of a braking mechanism. The most commonly used brake fluids are glycol ether-based liquids that may be blended with polyglycols. These liquids are hygroscopic and will absorb moisture from the atmosphere, which gradually reduces the boiling point of the brake fluid. If the boiling point of the brake fluid is lowered, bubbles of vapour may be produced during braking when friction causes the temperature of the brake fluid to rise. Vapour bubbles impair the functioning of the hydraulic mechanism and this represents a significant safety hazard. Additionally, increased levels of water in brake fluid can negatively impact the low temperature viscosity, the anti-corrosion performance and the elastomer compatibility of the brake fluid.
Several systems are used to determine whether the boiling point of a brake fluid has been compromised by absorption of water. The Equilibrium Reflux Boiling Point (ERBP) method (e.g. as specified by the US Federal Motor Vehicle Safety Standard (FMVSS 116)) is typically used to determine boiling point and Karl Fischer titration (e.g. according to the ASTM standard D1123) may be used to determine water content. These methods are accurate but are labour intensive and must be conducted by a skilled person in a laboratory. Moreover, the ERBP method requires a significant sample quantity making it impractical to apply to a brake fluid while present in a car braking system.
U.S. Pat. No. 5,785,425 and U.S. Pat. No. 5,380,091 both disclose hand held devices for detecting degradation in brake fluid quality that could be used by a car mechanic in a garage. A probe is inserted into the brake fluid and a display indicates the boiling point of the brake fluid to the user.
U.S. Pat. No. 4,869,596 discloses a method for checking the state of a brake fluid contained in a vehicle wherein the boiling point of the braking fluid is measured, the temperature of the braking fluid is measured, and these are compared with standard values. Changes in the brake fluid quality are indicated to the driver of the vehicle, so that the driver is informed of hazardous changes and can take appropriate action.
DE 3143589 discloses a device wherein a colour indicator is brought into contact with a brake fluid, and wherein a colour change indicates the water content of the brake fluid. DE 19838025 discloses brake fluid comprising a colour indicator such that the brake fluid changes colour as the water content of the brake fluid varies. FR 2664701 discloses another method wherein colour indicators are used to indicate the water content of brake fluid.
The present inventors have sought to provide an alternative method and device for detecting degradation in the quality of a brake fluid. Desirably the method and device reliably indicate the presence of water in the brake fluid, without the need for multiple complicated measurements. Most desirably, the method and device allow for detection of water whilst the brake fluid is still present in the braking system (i.e. there is no requirement to remove a sample of the brake fluid from the system so that it can be tested), and preferably the method and device allow for detection of water at or near a wheel cylinder where the presence of water in the brake fluid is most critical.